Four-component zinc-porphyrin/zinc-salphen nanorotor

Abstract

An off-axis supramolecular rotor was composed of four components: a zinc-porphyrin based stator with four phenanthroline stations and a zinc-salphen based rotator were self-assembled with DABCO and four copper(I) ions to furnish the rotor ROT-2 in quantitative yield. The DABCO serves as a connecting axle between the rotator and the stator, while the rotator is additionally connected to two copper(I)-loaded phenanthroline stations of the stator via its two pyridine terminals (N_py → [Cu(phen)]⁺). For the thermally activated rotation both N_py → [Cu(phen)]⁺ interactions have to be cleaved. Due to the high energy barrier of the rotation the slow motion was monitored by ROESY. The reduced speed (k₂₉₈ = 0.2 s⁻¹) was rationalised in terms of ground state stabilisation of the rotor as suggested by computational insight. Additional VT ¹H-NMR investigation was undertaken to study the motion of DABCO that is sandwiched between zinc porphyrin and zinc salphen. The diagnostic splitting of the geminal DABCO protons was rationalised on the basis of the asymmetry induced by the salphen that generates a diastereotopic environment. Computations reproduced the experimental NMR with excellent agreement.